1. Field of the Invention
The present invention relates to heretofore unknown membranes based on a vinyl alcohol polymer (hereinafter also referred to as PVA) and to methods for producing the same. More particularly, the invention relates to a PVA dialysis, osmosis or ultrafiltration membrane of heretofore unknown structure, and to methods for producing PVA membranes by preparing a PVA composition containing a polyalkylene glycol (hereinafter referred to as PAG) to coagulate the composition in a liquid bath and, thereafter if necessary, immersing the resultant shaped article in water at an elevated temperature.
2. Description of the Prior Art
The recent past several years have witnessed remarkable advances in separatory processes which utilize membranes possessing selective permeability. These membrane separation techniques have been utilized with some success in certain applications such as in the desalination of sea water and the treatment of blood in artificial kidneys. For some applications, however, the membranes currently in use are not completely satisfactory from all aspects, such as membrane permeability or other properties.
The membranes heretofore employed in various fields of separation engineering are mainly those formed of cellulose or its derivatives. Aside from these materials, a variety of other materials have been used in the preparation of membranes which have included PVA. One such PVA membrane is an aldehyde-crosslinked PVA membrane (Leininger et al, Trans, ASAIO 10 22, 1964). Another PVA based membrane is prepared by crosslinking allyl methacrylate as a polyfunctional monomer with radiation (M. Odian et al, Trans, ASAIO 14 5, 1968; Bruce S. Bernstein, J. Polymer Sci. Part A, 3 3405 1965). Still another reference shows a polyvinyl alcohol-chitosan blend membrane (S. Yoshikawa et al, a paper presented at the 20th Annual Meeting of the High Polymer Society of Japan). Yet another reference shows a polyvinyl alcohol graft-copolymer membrane (Y. Imai et al, JINKO-ZOKI (Artificial Organs) 2 147, 1973).
All of these membranes, and for that matter, all of the membranes generally prepared from a highly hydrophilic material such as PVA, while normally exhibiting satisfactory permeability, are inferior with regard to strength, and for this reason are usually not satisfactory for dialytic and other separation applications. On the other hand, membranes with sufficient strength tend to have poor permeability. The same situation applies equally to the instances where the membranes are used as flat membranes, and to the instances where the membranes are used in the form of hollow fibers which is more advantageous from a strength quality viewpoint. It is for these reasons, irrespective of whether a permeable material is used as a hollow fiber or as a flat membrane, that the commercial exploitation of PVA membranes has not materialized.
In the research leading to the present invention, the structures of various semi-permeable membrane materials (hereinafter referred to as simply membranes) have been studied. The study, with the aid of an electron microscope, a porosimeter and other instruments, has revealed that the conventional membranes have either an asymmetric structure made up of a relatively dense layer and porous layer having micropores not less than about 10 microns in diameter, or a homogeneous structure consisting exclusively of a comparatively dense layer whose porosity is not detectable by electronmicroscopy (.times. 100,000).
A need therefore continues to exist for PVA-based membranes which exhibit excellent permeation selectivity and superior mechanical strength and for methods of producing such membranes.